HYDRAULIC LIMES AND CEMENTS 155
7, Setting Time.—This is determined on the normal paste, and that
used in the preceding test may be used, being kept during the whole of
the tests in a moist chamber at about 15°. The test is made with Vicat's
needle (Fig. 8), which is a cylindrical steel needle of i sq. mm. section (1-13
mm. diameter), cut normally to the length and weighing, with the rest of
the moving parts, 300 grams. It is brought gently into contact with the
mortar to ascertain if it penetrates. The test is repeated at first every
minute and afterwards at gradually increasing intervals. The initial set
is taken as the time when the needle fails to reach the bottom of the test-
piece and the final set as that when the needle fails to enter to an appreciable
extent (about o-i mm.) ; times are measured from the moment when the
water is added to the cement for the gauging.
This procedure is that adopted by the Official Italian Regulations. When
setting is almost complete, it is well to detach the mould from the glass and test
on the lower surface of the mortar where it is more polished and homogeneous ;
with slow setting, it is useful to cover the mould with a glass to prevent drying.
Some advise, especially with very slow-setting products, the determination of
the time of setting under water.
The rise of temperature of the mortar during setting may give useful indica-
tions and is ascertained by means of a thermometer with its bulb immersed
in the paste ; temperature readings are taken at regular intervals and the tem-
perature-time curve traced.
8. Strength.—Only a brief reference will be made to these mechanical
tests, which are not usually made in chemical laboratories. Chief among
them are those of tensile and compression strengths.
According to the Official Italian Regulations, such tests are made in
some cases on a normal paste of the pure cement (see above, 6), but usually
on a normal mortar obtained by making an intimate mixture of I part of
the agglomerating material with 3 parts of sand and preparing a paste
from this and the necessary amount of water, to be determined by trial
(usually about 8% by weight of the mixture). The granules of the normal
sand to be used must pass through circular holes 1-5 mm. in diameter but
not through i mm. holes.
For tensile measurements briquettes are moulded in the shape of the
figure 8 and of definite form and dimensions, the minimum section in the
narrowest part being 5 sq. cm. The mortar is compressed into the mould
by 120 blows, delivered during 3 minutes, of a hammer weighing 2 kilos
falling from a height of 0-25 metre and developing energy amounting to
o-30-kilogram-nietre per gram of dry substance. The briquette is then
removed from the mould. The tests are usually made after 28 days (or 7
with rapid-setting cements), during the first of which (or the first two with
hydraulic limes) the briquettes are kept in moist air and during the remainder
in water. In some cases measurements are made after different periods
(in general, the strength after 28 days is about two-thirds of the value after
i year). The results are expressed in kilos per sq. cm. of the minimal
section and for a given material six briquettes are tested and the mean of
the four highest results taken.
With pure cement, the briquettes are made by hand with the help ofPublic Works, January 10, 1907.uperphosphate " (ibid., 1910, p. 297).ranhfonn into phosphoric at id the pyrti ami meta-pho.Hplwric